An offset printing press typically includes a plate cylinder carrying one or more printing plates. The printing plates have oleophilic surfaces defining an image area, and hydrophilic surfaces defining a non-image area. An inker applies ink to the printing plate which collects on the oleophilic surfaces to form an image which can be transferred to a blanket cylinder which transfers the image to media. By transferring the image from the printing plate onto a blanket roller, and then onto the media, the printing plate does not directly print the image on the media, hence the term offset printing.
The inker applies ink carried on one or more form rollers to the printing plate. When the form roller in the inker engages the printing plate, the ink film on the form roller contacting image areas on the printing plate is split such that approximately one-half of the thickness of the ink film is applied to the image area of the printing plate leaving approximately one-half the ink on the form roller causing a condition referred to as starvation. The ink film on the form roller contacting non-image areas on the printing plate remains on the form roller causing a condition called accumulation.
This combination of accumulation and starvation results in undesirable “ghosted” images and image repeats being formed on the final printed product. In order to minimize this problem, conventional inkers include a plurality of form rollers which each apply a small amount.
The printed product is monitored to determine when ink density has degraded beyond an acceptable level. In order to control the quality of the printing, conventional printer inkers also include a plurality of adjustable keys to control the amount of ink being applied to the form roller. These keys require constant adjustment to maintain the quality of the printed product.
One attempt to provide a keyless inker incorporated a reverse rotating roller in pressural indentation contact with a main form roller to meter the ink and erase the previous image on the form roller. This prior art inker provided an even film of ink on the printing plate, and inhibited the accumulation and starvation of ink on the form roller. This reverse roller imposed a counter rotating force to the main form roller which increased the power requirements for operating the printing press. In addition the friction caused by the counter-rotating roller generated a tremendous amount of heat that had to be “taken away,” resulting in more horse power and satellite refrigeration equipment at each printing assembly.
In U.S. Pat. No. 4,453,463, an inker is disclosed for a lithographic printing press in which dampening fluid is applied to a resilient form roller. A blade is mounted to remove the dampening fluid and excess ink directly from the resilient form roller surface. The form roller is rotated into the leading edge of the doctor blade, which is pressure indented to the form roller, and increases the power requirements for rotating the form roller. Furthermore, the blade has a tendency to damage the form roller resilient surface.
U.S. Pat. No. 4,527,479 discloses a method and apparatus for continuously using ink and dampening fluid in a printing system which includes removing ink and dampening fluid from a form roller after the form roller engages the printing plate. Unused printing ink and dampening fluid is removed from the form roller by an idler roller, and a scraping off means scrapes the mixture directly from the idler roller. The mixture is then returned to the reservoir. The ink and dampening fluid removed from the form roller are blended in the reservoir with fresh ink, and recirculated to a distributor line for application to the form roller. This concept works well for a printing press using a low viscosity news print ink which does not dry quickly onto a continuous media. However, for high quality multi colored sheet fed products, the circulation of ink and wash-up requirements is prohibitive.
Another attempt to solve the problem of ghosting is disclosed in U.S. Pat. No. 5,315,930 entitled “KEYLESS INKING SYSTEM FOR A PRINTING PRESS.” This patent discloses an inking system for a printing press having an ink injector for supplying ink under pressure, and a device for pumping and metering the ink flow in the injector. The ink injector supplies ink to a fountain roller having an outer brush surface. The fountain roller applies the ink to a pick up roller which transfers the ink through a series of rollers to an applicator roller. The applicator roller has a resilient surface, and applies the ink to two form rollers. A scraper roller engages the applicator roller to remove excess ink therefrom. A scraper blade scrapes ink from the scraper roller. Ink scraped from the scraper roller is transported to an ink reservoir, and is then recirculated using a pump to the ink injector. The inking system in U.S. Pat. No. 5,315,930 has multiple form rollers, and does not provide any means for removing excess ink from the form rollers. In addition, the inking system requires ink recirculation which requires a lengthy wash-up time.
All of the patents referred to above have sought to solve “ghosting,” starvation, and accumulation problems in inking systems. However, the solutions have complicated the printing press assemblies, require circulating the ink which complicates washing the inker for a color change, and can cause damage to the single form roller.